1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus using a semiconductor laser.
2. Description of the Related Art
Optical recording/reproducing apparatuses include an apparatus for recording information on an optical recording medium by radiating a light beam emitted from a semiconductor laser onto the recording medium, an apparatus for reproducing information recorded on a recording medium, and an apparatus having both functions of recording and reproducing information.
In general, in a semiconductor laser, a threshold current I.sub.th at which a light output from the laser rises abruptly is increased in accordance with the rise of temperature, on the basis of the following equation: EQU I.sub.th =k exp (T.sub.i /T.sub.o)
where k is a constant, a T.sub.i is a virtual junction temperature, and T.sub.o is a constant called "characteristic temperature" which is about 50.degree. to 200 [.degree. K]. In addition, in a region (oscillation region) where a drive current is higher than a threshold current, the variance degree (differential efficiency) of the light output in relation to the variance degree of current depends on temperatures. FIG. 3 is a graph showing the relationship between a drive current and a light output of a semiconductor laser. FIG. 3 shows two curves representing operation characteristics at different temperatures T.sub.1 and T.sub.2 (T.sub.1 &lt;T.sub.2) and threshold currents I.sub.th1 and I.sub.th2 of the corresponding operation characteristics. Because of this characteristic, even if the variance of drive current due to a temperature at a certain light output is corrected by an APC (Automatic Power Control) operation and thereafter constant-current drive is carried out in order to obtain a light output different from the light output obtained at the APC operation time, a desirable light output cannot be obtained owing to a variance in differential efficiency with respect to temperature. Consequently, the intensity of light beam radiated on a recording medium is varied.
Published Unexamined Japanese Patent Application (PUJPA) No. 1-196733, for example, discloses a prior art technique for solving the above problem. According to this technique, a drive current for a semiconductor laser is controlled by a signal from temperature detecting means for detecting the temperature within a drive unit, thereby controlling the intensity of a light beam radiated on the recording medium. This temperature control method will now be described with reference to FIG. 4. A divergent light beam emitted from a semiconductor laser 1 is changed to a parallel beam through a collimator lens 2. The parallel beam is converted through an objective lens 3. Thus, a beam spot 8 forms on an information recording surface of an optical recording medium 4 such as an optical card or an optical disc. A temperature sensor 6 senses a temperature within the drive unit. Detected data on temperature is sent to a controller 7. On the basis of the pre-set data on the relationship between the temperature within the drive unit and the intensity of light beam, the controller 7 generates a control signal for driving a semiconductor laser drive circuit 5. Upon receiving the control signal, the semiconductor laser drive circuit 5 supplies the semiconductor laser 1 with a constant current for enabling the laser 1 to output a light beam with an optimal intensity in relation to the present temperature within the drive unit. This sequence of operations are constantly carried out to keep the optimal intensity of the light beam of the semiconductor laser 1 in relation to the temperature within the drive unit.
According to this method, since the temperature within the drive unit is detected, the temperature variance of the semiconductor laser due to the temperature variance within the drive unit can be corrected. However, the time constant of the thermal diffusion in the drive unit is greater than that of the temperature variance of the semiconductor laser itself. Thus, this method cannot correct a temperature variance occurring within the semiconductor laser in a relatively short time period after a high-output driving operation such as a recording operation. As a result, the intensity of a light beam radiated on a recording medium varies.